CZ293256B6 - Method of producing carbide-free bainitic steel rail - Google Patents

Abstract

The present invention relates to a method of producing a wear and rolling contact fatigue resistant carbide-free bainitic steel rail, the method comprising the steps of hot rolling to shape a steel whose composition by weight includes from 0.05 to 0.50 percent by weight of carbon, from 1.00 to 3.00 percent by weight of silicon and/or aluminum, from 0.50 to 2.50 percent by weight of manganese, from 0.25 to 2.50 percent by weight of chromium, balance iron and incidental impurities, and continuously cooling the rail from its rolling temperature to ambient temperature naturally in air.

Description

A method for producing a rail of carbide-free bainitic steel

Technical field

The invention relates to carbide-free bainitic steels and to methods for producing such steels. In particular, the invention relates to carbide-free bainitic steels having increased wear and rolling fatigue resistance, from which, inter alia, track and crane rails, switches and crossings, railway wheels and special wear parts and plates can be produced.

BACKGROUND OF THE INVENTION

Most of the rails are still made of pearlitic steels. Recent research indicates that the demands placed on pearlitic steels for the production of track rails are already approaching the achievable limits of their material properties. Thus, there has been a need to search for alternative types of steels that have good wear resistance and rolling contact fatigue in conjunction with good toughness and weldability.

EP 0 612 852 A1 discloses a method for producing high strength bainitic steel rails with good rolling fatigue resistance, wherein the rail head is subjected to a discontinuous cooling program comprising accelerated cooling from the austenitic region to a dwell temperature of 500 to 300 ° C at 1 to 10 ° C per second and further cooling the rail head to an even lower temperature region. The rails produced in this way wear more easily than conventional pearlitic rails and thus exhibit increased rolling fatigue resistance. Indeed, the increased wear rate exhibited by the surface of the rail head ensures that the accumulated fatigue-induced damage will be eliminated before causing the failure. The physical properties exhibited by these rails are partially achieved by the accelerated cooling regime described above.

The solution proposed in EP 0 612 852 A1 differs significantly from the method according to the invention, in which a substantially increased wear resistance is achieved in rail steel along with excellent rolling fatigue resistance. Compared to pearlitic steels, these steels also exhibit increased impact toughness and ductility. The process according to the invention also eliminates the need for the complex discontinuous cooling regime of EP 0 612 852 A1.

Other similar documents describing complex discontinuous cooling regimes are GB 2 132 225, GB 2 071 144, GB 1 450 355, GB 1 417 330, US 5 108 518 and EP 0 033 600.

Track rails made of bainitic steel containing iron carbide have been proposed earlier. While fine ferrite plates (-0.2 to 0.8 µm wide) and high dislocation density of continuously cooled bainite give steels high strength, the presence of carbides between and in ferrite plates causes increased brittleness, which has hitherto largely prevented the commercial use of such steels.

It is known that the problems caused by the presence of harmful carbides can be largely avoided by the relatively large additions of silicon and / or aluminum (~ 1 to 2%) to low alloy steels. Assuming that the dispersed, unconverted austenite is both thermally and mechanically stable, the presence of silicon and / or aluminum in continuously transformed bainite steels promotes the formation of carbon-enriched austenite tensile regions instead of the ferrite plates of dispersed brittle cementite layers. It has been shown that unformed austenite, which remains after continuous cooling in the region of bainitic transformation temperatures, occurs either in the form of finely dispersed thin layers within the platelets or forms block regions between particles. While the thin film morphology has very high thermal and mechanical stability, the blocks may reshape

Carbon-enriched martensite, which does not contribute much to high fracture toughness. In order to guarantee good toughness, the ratio between the morphologies of thin films and blocks should be higher than 0.9, this can be achieved by carefully selecting the steel composition and heat treatment.

The result is a substantially carbide-free upper bainite microstructure composed of bainite ferrite, residual austenite and carbon-enriched martensite.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide carbide-free bainitic steels with substantially improved hardness parameters, those that exhibit obvious advantages over prior art track rail steels.

According to a first aspect, the present invention provides a process for producing a wear-resistant and fatigue-resistant, wear-free, fatigue-resistant rail 15 whose microstructure is substantially free of carbides, the process comprising the steps of hot rolling steel comprising 0.05 to 0.50 wt. % of carbon, 1.00 to 3.00 wt. % silicon and / or aluminum, 0.50 to 2.50 wt. and 0.25 to 0.25

2.50 wt. Chromium, the remainder being iron and random impurities, and continuous cooling of steel from the rolling temperature naturally in air.

The steel may further comprise one or more of the following additives: up to 3.00 wt. nickel; % to 0.025 wt. open; % to 1.00 wt. tungsten; % to 1.00 wt. molybdenum; % to 3.00 wt. copper; % to 0.10 wt. titanium; % to 0.50 wt. % vanadium and up to 0.005 wt. boron.

The carbon content of the preferred steel compositions may be from 0.10 to 0.35% by weight. The silicon content may be from 1.00 to 2.50% by weight. Also, the manganese content may be from 1.00 to 2.50 wt%, the chromium content may be between 0.35 wt% and 2.25 wt%. and the molybdenum content may be from 0.15 to 0.6% by weight.

According to a second feature, the invention provides wear and fatigue-resistant steel produced by the processes described in the preceding three paragraphs.

In another aspect, the invention provides a hot-rolled or improved-cooled rolling fatigue and wear resistant rail of bainitic steel 35 having an iron carbide-free microstructure, wherein the rail, after hot rolling, cools naturally naturally in air or by accelerated cooling.

The steels of the present invention exhibit increased levels of rolling fatigue, ductility, flexural fatigue life, and fracture toughness in conjunction with a similar or better resistance to rolling wear than existing heat treated pearlitic rails.

In certain circumstances, it is considered advantageous that the rail has an appropriate high wear rate that allows continuous accumulation of rolling fatigue damage on the rail head surface. An obvious way to increase the wear rate of a rail is to reduce its hardness. However, a significant reduction in rail hardness causes large plastic deformations that are undesirable on the rail head surface.

Thus, a new solution to this problem lies in the ability to produce a rail with a sufficiently high hardness / strength that will withstand excessive plastic deformation during operation and thereby maintain the desired rail shape while exhibiting a correspondingly high wear rate for continuous removal of rolling fatigue damage. . This is achieved according to the invention by means of a suitable steel composition by deliberately forming a small portion of the soft pro-eutectide ferrite of a substantially carbide-free bainitic microstructure.

-2GB 293256 B6

One of the advantages of processing the naturally air-cooled bainitic steels according to the invention over existing high-strength pearlitic steels consists in the elimination of heat treatment operations during the production of the rails and subsequent to their joining by welding.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a cross-sectional view of a bainitic steel rail without iron carbide according to the invention with hardness values indicated;

Fig. 2 is a schematic CCT diagram (ARA anisothermic decay of austenite) for carbide-free bainitic steel according to the invention;

Fig. 3 is a scanning electron microscope image showing the microstructure of carbide-free bainitic steel according to the invention;

Fig. 4 shows notch toughness-temperature curves for iron-free carbide-free bainitic steel according to the invention after rolling as compared to similar curves for ordinary carbon heat-treated pearlitic steel used to date for rails; rolling wear rates on hardness for steel samples made of carbide-free bainitic steel according to the invention;

Fig. 6 shows the durability for the carbide-free bainitic steel of the invention and the commercially available wear-resistant abrasive abrasive materials;

Fig. 7 is a graph of hardness versus melting distance of a butt-welded, carbide-free bainitic steel sheet according to the invention; and Fig. 8 is a hardenability curve for carbide-free bainitic steel after rolling according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The primary object of the invention is to provide a high-strength wear-resistant and fatigue-resistant rolling contact microstructure in the rail head, which consists mainly of carbide-free bainite, some carbon-rich martensite and residual austenite. In practice, however, it has been found that such a high-strength microstructure is also located in the web and foot of the just rolled rail. A typical Brinell hardness (HB) profile for a 113 lb / yd rail cut is shown in Figure 1.

The high strength of the head, web and foot of the rail ensures high resistance to rolling fatigue and bending fatigue when using the rail.

This and other desirable objectives are achieved by careful selection of the steel composition and continuous cooling of the steel in air after hot rolling.

The additive ranges for the steels according to the invention are given in Table A.

-3GB 293256 B6

Table A

Element Content Range (wt%) Carbon 0.05 to 0.50 Aluminum / Silicon 0.50 to 3.0 Manganese 0.5 to 2.5 Nickel / Copper to 3.0 Chrome 0.25 to 2.5 Tungsten to 1.0 Molybdenum do 1,00 Titanium to 0.10 Vanad do 0,5 Boron to 0.0050 Residue Iron & random dirt

Within the above ranges, the contents of the individual additives may vary depending, inter alia, on the desired hardness, ductility, etc. However, all steels are naturally bainitic and free of carbides. Thus, the preferred carbon content may fall within the range of 0.10 to 0.35 wt%. The silicon content can be from 1 to 2.5% by weight, the manganese content from 1 to 2.5% by weight, the chromium content can be from 0.35 to 2.25% by weight. and a molybdenum content of from 0.15 to 0.6 wt.

The steels according to the invention generally have hardness values between 390 and 500 HV30, but it is also possible to produce steels with lower hardnesses. Typical values of hardness, wear rate, elongation and other physical parameters can be found in the attached Table B for the eleven steel samples of the invention.

Figure 2 is a schematic CCT (ARA) diagram. The addition of boron slows the conversion of austenite to ferrite, so that when the bainite is cooled continuously, it forms in a wide range of cooling rates. In addition, the bainite curve has a flat peak, so that the conversion temperature is almost constant over a wide range of cooling rates. This results in only small variations in strength over relatively large, air-cooled or fast-cooled areas.

The steels listed in Table B were rolled to 30 mm thick plates (cooling rates of 30 mm plates are close to those at the center of the rail head) from square ~ 125mm ingots and freely air cooled from a final rolling temperature of -1000 ° C to ambient temperature. The microstructure thus formed, as shown in Figure 3, consists essentially of a mixture of carbide-free bainite and residual austenite with varying amounts of carbon-rich martensite.

A comparison of some of the mechanical properties achieved on 30 mm thick bainitic steel test plates after rolling with the typical values of currently produced heat treated (MHT) rails is shown in the following table:

Rail type R0.2 [N / rnrn ² ] Rm [N / mm ^2] Prodl. [%] Cross-section reduction [%] HV30 CVN at 20 ° C [J] Kic At -20 ° C [MPam ^{l / 2} ] Speed wear [mg / m slip] (contact stress 750 N / mm ² ) MHT 800-900 1150-1300 9-13 20-25 360-400 3-5 30-40 20-30 Bainitická 730-1230 1250-1600 14-17 40-55 400-500 20-39 45-60 3-36

The properties of the 30 mm thick bainitic steel plate after rolling are characterized by a significant increase in the strength and hardness limits compared to the heat-treated pearlitic rail,

-4GB 293256 B6 Increasing notch toughness (CVN - Charpy V-notch, Charpy test, V-notch) from 4 J to typical 35 J at 20 ° C. Notch toughness CVN for two rolled bainitic steels (0.22% C, 2% Cr, 0.5% Mo, without B; and 0.24% C, 0.5% Cr and 0.0025% B) together with ordinary carbon heat treated pearlitic rails are shown in Fig. 4. It is clear that both bainitic steels retain high notch toughness down to temperatures of about -60 ° C.

The rolling contact wear of 30 mm thick bainitic steel plates after rolling in laboratory tests at a contact stress of 750N / mm ² appears significantly less than that of existing pearlitic heat treated rails, as shown in Figure 5.

Tests carried out in connection with the steels of the present invention have further shown that the composition of bainitic steels also guarantees high abrasion wear resistance compared to the mild steel standard against round quartz abrasives with a relative service life of about 5.0. Giant. 6 indicates that such durations are better than those of many commercially available wear resistant materials such as Abrazo 450 and 13% Cr martensitic steel.

It has been found that the fracture toughness (resistance to the spread of an existing crack) of 30 mm thick bainitic steel plates after rolling is significantly higher, between 45 and 60 MPam, than for heat treated pearlitic rails, which usually 30 to 40 MPam ^1/2 .

As shown in Fig. 7, rolled 30 mm thick bainitic steel plates are easily butt-weldable, with naturally air-cooled hardness in critical affected (HAZ) weld areas, butt-welded slabs correspond to the hardness of the base material, or even slightly higher .

As shown in the diagram in Fig. 8, the 30 mm thick bainitic steel plates after rolling have high hardenability with an almost constant hardness in the range from the hardened face between 1.5 and 50 mm, corresponding to cooling rates between 225 and 2 ° C. s at 700 ° C.

Although the invention has been described with particular reference to rails, other recommended fields of application for the steels of the invention include crane rails, rail switches and crossings (both cast and machined), rail wheels, particularly abrasive wear resistant parts and plates, and special support structures.

Claims (8)

PATENT CLAIMS CLAIMS 1. A process for the manufacture of a wear-free and fatigue resistant rail of a carbide-free bainitic steel, characterized in that it comprises the steps of hot rolling to form a steel having a mass composition of from 0.05 to 0.50% carbon, from 1 00 to 3.00% silicon and / or aluminum, from 0.50 to 2.50% manganese, from 0.25 to 2.50% chromium, from 0 to 3.00% nickel, from 0 to 0.025% sulfur , from 0 to 1.00% tungsten, from 0 to 1.00% molybdenum, from 0 to 3% copper, from 0 to 0.1% titanium, from 0 to 0.50% vanadium and from 0 to 0.005% boron, the rest forms iron and accidental impurities; and the step of continuously cooling the rail from the rolling temperature to ambient temperature naturally in air. Method according to claim 1, characterized in that the carbon content of the rail is from 0.10 to 0.35% by weight. -5GB 293256 B6 The method of claim 1, wherein the silicon content is from 1.00do 2.50 wt. The method of claim 2, wherein the silicon content is from 1.00do 2.50 wt. The method of claim 1, wherein the manganese content is from 1.00do 2.50 wt.%, The chromium content is between 0.35 to 2.25 wt. and the molybdenum content is from 0.15do 0.60 wt. 6. The process of claim 2 wherein the manganese content is from 1.00 to 1.00 2.50 wt.%, The chromium content is between 0.35 to 2.25 wt. and the molybdenum content is from 0.15 to 0.60% by weight. 7. The process of claim 3 wherein the manganese content is from 1.00 to 1.00 2.50 wt.%, The chromium content is between 0.35 to 2.25 wt. and the molybdenum content is from 0.15 to 0.60% by weight. The process according to claim 4, wherein the manganese content is from 1.00 to 2.50 wt.%, The chromium content is between 0.35 to 2.25 wt. and the molybdenum content is from 0.15 to 0.60% by weight.